The present invention relates to methods and apparatuses for monitoring and controlling mechanical and/or chemical-mechanical planarization of semiconductor wafers, field emission displays and other types of microelectronic substrate assemblies.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly.
FIG. 1 is a schematic isometric view of a web-format planarizing machine 10 for planarizing a microelectronic substrate assembly 12. The planarizing machine 10 has a table 11 with a rigid panel or plate to provide a flat, solid support surface 13 for supporting a portion of a web-format planarizing pad 40 in a planarizing zone xe2x80x9cA.xe2x80x9d The planarizing machine 10 also has a pad advancing mechanism including a plurality of rollers to guide, position, and hold the web-format pad 40 over the support surface 13. The pad advancing mechanism generally includes a supply roller 20, first and second idler rollers 21a and 21b, first and second guide rollers 22a and 22b, and a take-up roller 23. As explained below, a motor (not shown) drives the take-up roller 23 to advance the pad 40 across the support surface 13 along a pad travel path Txe2x80x94T. The motor can also drive the supply roller 20. The first idler roller 21a and the first guide roller 22a press an operative portion of the pad against the support surface 13 to hold the pad 40 stationary during operation.
The planarizing machine 10 also has a carrier assembly 30 to translate the substrate assembly 12 across the pad 40. In one embodiment, the carrier assembly 30 has a head 32 to pick up, hold and release the substrate assembly 12 at appropriate stages of the planarizing process. The carrier assembly 30 also has a support gantry 34 and a drive assembly 35 that can move along the gantry 34. The drive assembly 35 has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the head 32 via another shaft 39. The actuator 36 orbits the head 32 about an axis Bxe2x80x94B to move the substrate assembly 12 across the pad 40.
The polishing pad 40 may be a non-abrasive polymeric pad (e.g., polyurethane), or it may be a fixed-abrasive polishing pad in which abrasive particles are fixedly dispersed in a resin or another type of suspension medium. A planarizing fluid 50 flows from a plurality of nozzles 49 during planarization of the substrate assembly 12. The planarizing fluid 50 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the surface of the substrate assembly 12, or the planarizing fluid 50 may be a xe2x80x9ccleanxe2x80x9d non-abrasive planarizing solution without abrasive particles. In most CMP applications, abrasive slurries with abrasive particles are used on non-abrasive polishing pads, and non-abrasive clean solutions without abrasive particles are used on fixed-abrasive polishing pads.
In the operation of the planarizing machine 10, the pad 40 moves across the support surface 13 along the pad travel path Txe2x80x94T either during or between planarizing cycles to change the particular active portion of the polishing pad 40 in the planarizing zone A. For example, the supply and take-up rollers 20 and 23 can drive the polishing pad 40 between planarizing cycles such that a point P moves incrementally across the support surface 13 to a number of intermediate locations I1, I2, etc. Alternatively, the rollers 20 and 23 may drive the polishing pad 40 between planarizing cycles such that the point P moves all the way across the support surface 13 to completely remove a used portion of the pad 40 from the planarizing zone A. The rollers may also continuously drive the polishing pad 40 at a slow rate during a planarizing cycle such that the point P moves continuously across the support surface 13. Thus, the polishing pad 40 should be free to move axially over the length of the support surface 13 along the pad travel path Txe2x80x94T.
CMP processes should consistently and accurately produce a uniform, planar surface on substrate assemblies to enable circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 xcexcm. Focusing photo-patterns to such small tolerances, however, is difficult when the planarized surfaces of substrate assemblies are not uniformly planar. Thus, to be effective, CMP processes should create highly uniform, planar surfaces on substrate assemblies.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate assembly as quickly as possible. The throughput of CMP processing is a function of several factors; one of which is the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate assembly is planar and/or when enough material has been removed from the substrate assembly to form discrete components (e.g., shallow trench isolation areas, contacts, damascene lines). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is xe2x80x9cunder-planarized,xe2x80x9d or too much material can be removed from the substrate assembly if it is xe2x80x9cover-polished.xe2x80x9d For example, over-polishing can completely destroy a section of the substrate assembly or cause xe2x80x9cdishingxe2x80x9d in shallow-trench-isolation structures. Thus, it is highly desirable to stop CMP processing at the desired endpoint.
One method for determining the endpoint of CMP processing is described in U.S. Pat. No. 5,036,015 issued to Sandhu (xe2x80x9cSandhuxe2x80x9d), which is herein incorporated by reference. Sandhu discloses detecting the planar endpoint by sensing a change in friction between a wafer and the polishing medium. Such a change of friction may be produced by a different coefficient of friction at the wafer surface as one material (e.g., an oxide) is removed from the wafer to expose another material (e.g., a nitride). In addition to the different coefficients of friction caused by a change of material at the substrate surface, the friction between the wafer and the planarizing medium can change during CMP processing because the surface area of the substrate contacting the polishing pad changes as the substrate becomes more planar. Sandhu discloses endpointing CMP processing by measuring the electrical current through the platen drive motor and/or the drive motor for the substrate holder to estimate the friction between the substrate assembly and the polishing pad and detecting a change in the motor current.
Although Sandhu discloses a viable process for endpointing CMP processing, the change in electrical current through the platen and/or drive motor may not accurately indicate the endpoint of a substrate assembly. For example, because the friction between the substrate assembly and the planarizing medium can increase or decrease throughout a planarizing cycle according to the topography of the substrate assembly and the materials, it may be difficult to identify a definite change in the motor current indicating that the endpoint has been reached. Moreover, other parameters that are not related to the drag force between the pad and the substrate assembly, such as friction losses and other power losses in the motors, gearboxes or other components, may change the current draw through the motors independently from the drag force or have a significantly greater magnitude than the drag force. The change in current through the drive motors, therefore, may not accurately reflect the drag force between the wafer and the polishing pad because the drag force is not the only factor or even the primary factor that influences the current draw. Thus, it would be desirable to develop an apparatus and method for more accurately endpointing planarization of microelectronic substrate assemblies.
It is also difficult to monitor or control other aspects of CMP processing because the chemical effects and mechanical effects separately influence the polishing rates and other variables of CMP processing. For example, the uniformity of the planarizing process and the polishing rate is a function of downforce, chemical effectiveness of the planarizing solution, distribution of abrasive particles, relative velocity between the substrate assembly and the polishing pad, pad condition, platen temperature and several other variables. As such, when unexpected results occur, the large number of variables in CMP processing makes it difficult to determine the source of a problem. Therefore, it would be desirable to develop an apparatus and method that accurately endpoints planarization of microelectronic substrate assemblies and simplifies controlling other process variables.
The present invention is directed toward mechanical and/or chemical-mechanical planarization of semiconductor wafers, field emission displays and other microelectronic substrate assemblies. One method of planarizing a microelectronic substrate assembly in accordance with the invention includes pressing a substrate assembly against a planarizing surface of a polishing pad at a pad/substrate interface defined by a surface area of the substrate assembly contacting the planarizing surface. The method continues by moving the substrate assembly and/or the polishing pad with respect to the other to rub at least one of the substrate assembly and the planarizing surface against the other at a relative velocity. As the substrate assembly and polishing pad rub against each other, a parameter indicative of drag force between the substrate assembly and the polishing pad is measured or sensed at periodic intervals. The measured drag force can be used to generate a plot of work or force versus time. The work/force versus time plot is then integrated to determine an estimated total work exerted at the pad/substrate interface. The planarizing process is terminated when the estimated change in thickness of the substrate assembly is at least approximately within a desired range for endpointing the substrate assembly according to a predetermined relationship between work and change in substrate assembly thickness.